---
title: "Port 3 of 8051"
subject: "Embedded System"
chapter: 9
key: "semester_6/embedded_system:8051-port3-explain"
source: https://www.bctengineer.com/q/semester_6/embedded_system/8051-port3-explain
site: BCT Engineering
---

# Port 3 of 8051

Embedded System · Chapter 9: Microcontrollers in Embedded Systems

- Asked in: 2076 Baisakh, 2 marks
- Video: [8051 Microcontroller Port 3 Explained | Pin Functions, RXD, TXD, INT0, T0, PSEN | Hindi |TechStudy|](https://www.youtube.com/watch?v=e0H7T49Gz8c) — Tech Study
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/embedded_system/8051-port3-explain
- Whole chapter: https://www.bctengineer.com/md/semester_6/embedded_system/chapter-9

## Question

Explain about PORT 3 of 8051 microcontroller.

## Short answer

Port 3 is an 8-bit bidirectional I/O with internal pull-ups. Each pin has an alternate function: P3.0 (RXD), P3.1 (TXD) serial, P3.2 (INT0), P3.3 (INT1) interupts, P3.4 (T0), P3.5 (T1) timer inputs, P3.6 (WR) and P3.7 (RD) exteranal memory strobes. Write 1 to read input.

## Answer

Port 3 (P3.0-P3.7, pins 10-17) is an 8-bit bidirectional I/O port with internal pull-ups. Besides general I/O, every pin has a special alternate function that is used when the corresponding feature is enabled:

| Pin | Alternate function |
|---|---|
| P3.0 | RXD, serial input |
| P3.1 | TXD, serial output |
| P3.2 | INT0, external interrupt 0 |
| P3.3 | INT1, external interrupt 1 |
| P3.4 | T0, Timer 0 external input |
| P3.5 | T1, Timer 1 external input |
| P3.6 | WR, external data memory write |
| P3.7 | RD, external data memory read |

To use a pin as an input, 1 must be written to it first.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which Port 3 pins serve as the serial input and output?
   A. P3.0 = TXD, P3.1 = RXD
   B. P3.0 = RXD, P3.1 = TXD
   C. P3.2 = RXD, P3.3 = TXD
   D. P3.6 = RXD, P3.7 = TXD
   Answer: B. P3.0 is RXD (serial input) and P3.1 is TXD (serial output). (Key point: Serial pins)

2. Which Port 3 pins carry the external data memory write and read signals?
   A. P3.4 and P3.5
   B. P3.2 and P3.3
   C. P3.6 (WR) and P3.7 (RD)
   D. P3.0 and P3.1
   Answer: C. P3.6 is WR and P3.7 is RD for external data memory access. (Key point: External memory strobes)

3. What must be done before using a Port 3 pin as an input?
   A. Write 0 to it
   B. Write 1 to it
   C. Set EA high
   D. Clear PSW
   Answer: B. Writing 1 releases the pin to its pull-up so an external signal can drive it. (Key point: Using a pin as input)

4. Which pins are the external interrupt inputs INT0 and INT1?
   A. P3.2 and P3.3
   B. P3.4 and P3.5
   C. P3.0 and P3.1
   D. P3.6 and P3.7
   Answer: A. P3.2 is INT0 and P3.3 is INT1; P3.4 and P3.5 are the timer inputs T0 and T1. (Key point: External interrupts)

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